It is known to provide heat exchangers which have at least two cylindrical shells lying one within the other and defining a pair of annular chambers disposed one within the other, for power plants having steam-driven turbines for the generation of electrical energy.
Such heat exchangers have been used in the past for the heating of feed water of the power plant. In order that the flow paths for the steam and feed water may be kept as short as possible, because the heat exchanger is generally disposed directly beneath the turbine on the turbine mount of the power plant and its dimensions are limited between fittings necessary to communicate with other parts of the power plant, the heat exchanger is provided with annular chambers, as described, to be traversed by the steam and with tube bundles or the like which can be traversed by the feed water.
The feed water flows through the heat exchanger at high pressure and with high speed and hence pressure drops must be minimized within the heat exchanger. Furthermore, the outer dimensions of the heat exchanger, the locations of the inlet and outlet fittings on the heat exchanger structure and the internal volume of the heat exchanger are generally fixed by the requirements for positioning of the heat exchanger in relation to the turbine as described. Usually the feed water must be led from the same side of the heat exchanger as it is introduced.
It has already been pointed out that feed water preheaters for power plants of the aforementioned type have been provided heretofore with inner and outer annular chambers each receiving a tube bundle of U-shaped bent tubes. The feed water is fed to the heat exchanger at one of its end faces, is passed through the U-shaped bent tubes in the next annular chamber in a foward and return direction, is then conducted into the outer annular chamber and trasverses the U-shaped tubes therein in the forward and reverse directions before being discharged from the heat exchanger.
The steam at elevated pressure is supplied at the other end face of the heat exchanger in order to insure a uniform heat transfer to the U-shaped tubes in the inner chamber. In addition, complicated inserts and devices must be used in the annular chambers to insure effective steam distribution.
Since, however, the distance between the inlets and outlet for the heat exchanging fluids is fixed (by reason of the location at which the heat exchanger must be installed with relation to the turbine set), the heat exchanger has a limited length. Moreover, the flow velocity of the feed water is generally so high that errosion damage can occur, leading to breakdown of the system. The high inlet velocity of the feed water is also accompanied by a high and disadvantageous pressure drop.